Self made voltage regulator doesn't work

[scofmb]

Hello

I'm trying to make a voltage regulator, and it work but under certain conditions it doesn't work as it have to.

I put my reference so i get 16V as output

At low current, i can get 16V output. (15mA)
At high current, i can only get 12V output (100mA)

I put my reference so i get 10V as output
I can get any current i want.

The IN voltage to be regulated is 17V, the opamp is powered with +-12V

I don't know where is the flaw in my "more than common" design
In the simulators (Microcap, multisim) it work ok, but in real-life it doesn't

Thank in advance

Ramiro
PD: Sorry for my bad english

 

[mister_rf]

So you have only 17V on the input and there’s a need to obtain a 16V output?
I think TIP105 have 2V-2.5V collector-emitter saturation voltage and also the C5 capacitor is too small.
R6 is too small, try 470-680 ohms.

 

[scofmb]

Nop, if it were a NPN would get a drop of 2 and 2,5V.. a PNP has a drop of 0,7-1V.

Its called low-dropout voltage regulator (the circuit).. i didn't invent nothing in this circuit.

And i changed the resistence as you said and keep working the same...

Without taking current i get 15,8V
everytime i put a higher load i get lower voltage... so, lets said i take 150mA (i get 12V output).

but, if the output voltage is regulated below 12V, i can take any current i want.

 

[mister_rf]

Try to change the darlington transistor with a normal one, like BD236-238, BD434-438 series.

 

[FvM]

I'm not motivated to comment other suggestions, just a few remarks. I understand, that this is a simulation only, not a real circuit. In a real circuit, I would fear overload of Q1 that isn't able to withstand 1,6 W dissipated power it must handle at maximum with the values given in the schematic. This would not happen in normal operation, but with the output transistor near saturation.

I also understand, that the discussed problem (output voltage too low) is observed with the values given in the schematic, not with different load or input voltage. In this case, I wonder, if V1 may have a hidden inner resistance, that exist as an option with MultSim.

If the observations have been made with a real circuit or with different part dimensioning, I would like to see the real conditions. For an analysis, the node voltages in limited operation point would be interesting.

The circuit also may have stability issues, that would reveal in a transient analysis or in a real circuit.

 

[scofmb]

First, i tried normal PNP but doesn't work also..

and also, in simulation everything works like a charm, the problem is the real-life circuit.. (this is not my final design, just the topology )

And i'm still trying to find where the prob

Haven't had too much time either, studying..

Thanks

 

[FvM]

If the problem is with real circuit, the nature of the input voltage is one interesting point. Is it actually a DC voltage or filtered rectifier output? If so, how much are the DC and AC part, that can be measured at the input node 3? Which voltage level can be measured at node 2 and 8.

Actually 100 mA is a small load for TIP105, it should handle it with < 1 V voltage drop.

 

[mister_rf]

I think you need to add some capacitor to filter the voltage applied in the base of Q1.
Try something like 0.22-0.47uF non-polarized capacitor. This will solve the stability issues.